Hi,I have a 765M gpu taken from a Dell Alienware laptop that others have successfully installed in their 2011 iMacs but I can't get mine to POST. Details are in this thread https://forums.macrumors.com/threads/2011-imac-graphics-card-upgrade.1596614/page-34 and post #835 there by johnrak talks about modifying the heatsink pipe enclosure to avoid a coil. I just posted this my same query there actually but I know you guys are far more "up to speed" on these things so I am asking here again. My post there is #863 which is as follows:

Can someone help me please? Regarding post #835 in this thread, that is for a 21.5" iMac, Johnrak cut the casing of the heatsink piping back to avoid one of the two coils(?) on the 765M. When I installed the same card in my 27" iMac, it did not seem that the two were interfering with each other, although there may have been some contact. I modified the X-bracket that came with the card to allow use of the Apple screws to fix the heatsink to the video card. In short, the coil-heatsink proximity is the only physical anomaly I can see. When I assembled for testing, the machine would not pass POST (Power On Self Test). The 765M card has since been tested in another computer and it is fully functional. The iMac functions perfectly with the factory 6770M card. Any ideas? I can only think of applying thermal paste to the coil where it possibly contacts the heatsink pipe enclosure but have doubts that this is the cause of it failing to POST.

Yes I noticed that the hole in the middle of the AMD X-bracket is smaller than that in the nVidia one. The nVidia one came with bushes on though so I had to remove those to use the screws that fix it to the Apple heatsink.

The piping as it leaves the aluminium heatsink block has a slight flexibility in it, so even if in a perfect situation it touches components on the 765m (a square block, a coil(?)), it only rests on it, not applying any force, and there is no electrical conductivity that I can see. SMCs AFAIK are sealed components and conductivity is limited to the solder contacts, isn't it?

I suppose it be a good idea to apply thermal paste to components that may/do touch the heatsink assembly?

When I opened up the iMac to swap the GPU, I reset the NVRAM (cmd-option-P-R) first, but it just occurred to me that doing it again whilst trying to POST may help it along?

I had no photos before but I just had another attempt at swapping the GPU and took some. Here they are:

1. Orig. 6770M fixed to mainboard with heatsink attached2. Orig with heatsink next to 765M3. 765M is about 30mm longer than original 6770M4. 765M fixed to heatsink with nVidia X-bracket5. Other side of 765M + heatsink. I spread thermal paste on one coil in lower right but it hardly touches the heatsink pipe enclosure6. Hardly touching7. 765M assembled in the iMac with HDD to its right. The longer length of the card is not affected by anything, it is in free space8. A bank of 4 LEDs that seem to be used for video debugging (I reduced the photo size to conform to site rules so it is low quality and hard to see)

I replaced the orig card with the 765M and it pipped 3x repeatedly on test. I swapped back to the orig card and same 3 pips. I put the 765M again and it made 1 pip, I noticed only 1 LED was lit, LED501 the left hand one. I swapped the orig card back in and it made 1 pip with 1 LED, I put some RAM in and it booted normally. Seems 1 pip means "put RAM in." It was late and getting dark so I left it at that. Perhaps I just need to put RAM in with the 765M? Shame today ended like that. When I have time I'll have another go.

Oh, and after waking up wishing I had a 27" 2011 iMac to chop a hole out of I found a 27" iMac 50 ft from my parking space.

So, will make this work. But, man, Hollywood is full of wackjobs. They smashed the bejeezus out of a 27" iMac. Screen in a messy state, etc. But all of their personal info well and good on the internal HD.

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